This invention is directed to the selective hydrogenation of dienes to monoolefins, particularly to the selective hydrogenation of cyclopentadiene to cyclopentene. More specifically, it is directed to a process wherein cyclopentene is prepared which comprises selectively hydrogenating cyclopentadiene in a liquid phase by contacting cyclopentadiene with hydrogen in the presence of a hydrogenation catalyst comprising (1) a soluble nickel compound, (2) an aluminum trialkyl compound or a lithium alkyl compound, and (3) at least one cocatalyst compound selected from the group consisting of H.sub.2 O; NH.sub.3 ; ROH where R is an alkyl or a halogenated alkyl radical containing from 1 to 20 carbon atoms; ##STR4## where R.sub.1, R.sub.2 and R.sub.3 may be hydrogen, or an alkyl radical containing from 1 to 6 carbon atoms; ##STR5## where R.sub.1, R.sub.2 and R.sub.3 may be hydrogen or halogen, or an alkyl radical containing from 1 to 6 carbon atoms; R.sub.1 OR.sub.2 wherein R.sub.1 and R.sub.2 may be the same or different alkyl radicals containing from 1 to 6 carbon atoms; and ##STR6## wherein R.sub.1 may be alkyl or an aromatic radical containing from 1 to 8 carbon atoms and R.sub.2 may be hydrogen or an alkyl or aromatic hydrocarbon radical containing from 1 to 8 carbon atoms.
At the present time, substantial amounts of cyclopentadiene, usually as dicyclopentadiene, are available as a byproduct from the steam cracking of naphtha to produce primarily ethylene. Cyclopentene has been found to be useful as a monomer for the formation of general purpose elastomers by ring opening polymerization of cyclopentene. Therefore, it is desirable to convert a portion of the excess cyclopentadiene available into a more valuable raw material, such as cyclopentene.
The hydrogenation of cyclopentadiene to cyclopentene is not new. For instance, in U.S. Pat. No. 2,360,555, issued Oct. 17, 1944, there is disclosed a selective hydrogenation of one of the two conjugated double bonds of a cyclic diolefin to produce the corresponding cyclic monoolefin which is accomplished by conducting the hydrogenation in the liquid phase in the presence of an active hydrogenation catalyst, under moderate hydrogen pressure, such as 2 to 5 atmospheres absolute, and at relatively low temperatures, such as from 0.degree. to 40.degree. C. and even up to 100.degree. C., using substantially less than the stoichiometric amount of hydrogen theoretically required to completely reduce the cyclic diene to the corresponding cyclic monoolefin. The catalyst therein disclosed is a pyrophoric nickel metal catalyst, such as Raney nickel.
In U.S. Pat. No. 3,819,734, issued July 25, 1974, there is disclosed the hydrogenation of cyclopentadiene to cyclopentene by bringing cyclopentadiene into contact with a catalyst consisting essentially of (1) nickel, on a magnesium or zinc oxalate support, (2) a ligand selected from the group consisting of trimethyl phosphine, triethyl phosphine, methyl ethyl propyl phosphine, trimethyl phosphite, triethyl phosphite, tributyl phosphite, triphenyl phosphite, etc., while in the presence of hydrogen, at temperatures from 0.degree. C. and at pressures from 0 to 1000 pounds per square inch gauge.
In U.S. Pat. No. 3,994,986, issued Nov. 30, 1976, there is disclosed the preparation of cyclopentene from cyclopentadiene by hydrogenating cyclopentene with hydrogen gas at a ratio of 1 to 1.5 moles of hydrogen per mole of cyclopentadiene in the presence of a palladium catalyst on a carrier. Also, see U.S. Pat. No. 4,108,911, issued Aug. 22, 1978.
Also, in U.S. Pat. No. 3,857,894, issued Dec. 31, 1974, there is disclosed the hydrogenation of cyclopentadiene to cyclopentene in the presence of a palladium catalyst and a small amount of an aqueous solution of zinc salt having a water/zinc ratio of at least 1/1 by weight.
Also, see German Patent No. 2,327,230.
The cyclopentadiene employed in the formation of cyclopentene by hydrogenation is usually obtained by depolymerizing or cracking dicyclopentadiene. In order to obtain cyclopentadiene for the hydrogenation of this invention, the depolymerization of dicyclopentadiene is accomplished by heating the dimer at a temperature above 150.degree. C. under atmospheric pressure in a conventional cracking apparatus. The depolymerized material should be hydrogenated without substantial delay because it is also known that redimerization will occur upon standing.